1. Field of the Invention
This invention relates to an ophthalmologic apparatus for automatically effecting the alignment of an eye examining portion with an eye to be examined.
2. Description of Related Art
In a method of aligning with an eye to be examined in a conventional eye refractive power measuring apparatus for projecting a measuring beam onto the pupil of the eye to be examined, and effecting examination by reflected light from the fundus thereof, alignment is effected on the basis of the vertex of the cornea of the eye to be examined. Depending on the eye to be examined, however, there is a case where the pupil thereof and the vertex of the cornea are eccentric from each other, and when the eccentricity is great, the beam necessary for measurement is eclipsed by an iris and sometimes it becomes difficult to obtain a correct measurement value.
Also, for the accurate measurement of refractive power, it is better in accuracy to measure the refractive power at the center of the pupil which is the original visual line and therefore, there is known an apparatus as shown in Japanese Patent Application Laid-Open No. H9-66027 wherein the measurement of the refractive power is effected with a measurement optical axis aligned with the center of the pupil. In this example of the prior art, however, when the pupil of an eye to be examined is large, the eyelid becomes liable to cover the area of the pupil, and when the eyelid covers the area of the pupil, it becomes difficult to find the accurate central position of the pupil by such a simple calculation as finds the centroid of the pupil to thereby find the center of the pupil, and there is the undesirable possibility that measurement is effected at a position differing from the center of the pupil.
Also, depending on the manner in which the eyelid covers the area of the pupil, there is the problem that the detected position of the center of the pupil does not become constant, and during each measurement, the position measured changes and the measurement value is not stable. Further, there is the problem that to find the center of the pupil accurately with the eyelid covering the area of the pupil, complicated calculation becomes necessary and the calculation time becomes long and also the measurement time becomes long, and this applies an excess burden to an examinee.
Also, when the pupil of the eye to be examined is small and the diameter thereof is approximate to a measurable minimum pupil diameter, there is the possibility that even if alignment shift is within an allowable range, depending on the shift, the measuring beam is eclipsed by the iris. When the measuring beam is eclipsed by the iris, there is the problem that a measurement error will occur or a measurement value of low reliability will result.
Also, if the accuracy of alignment is made severe, much time is required of the alignment and therefore, the allowable range is set within a range which does not affect the measurement value. Usually, this allowable range of alignment is fixed, but as described in Japanese Patent 3161544, there is known an apparatus which can manually change the accuracy of alignment.
Also, in an apparatus described in Japanese Patent Application Laid-Open No. H11-19040, design is made such that measurement is effected with alignment adjusted to a position as nearest as possible to the vertex of the cornea at which the measuring beam is not eclipsed by the edge of the iris. In the above-described examples of the prior art, however, there is the problem that from the completion of the alignment till the completion of the measurement, much time is required of fog operation or the like which eliminates the adjusting power of the eye to be examined, and in the meantime, the eye to be examined moves, or when fog operation is effected, the diameter of the pupil changes, whereby even if upon completion of the alignment, the measuring beam is not eclipsed by the iris, during the actual measurement, the measuring beam covers the iris and faulty measurement occurs.